The present invention relates to a nonaqueous liquid electrolyte and a nonaqueous liquid electrolyte secondary battery
Currently, a lithium ion secondary battery has been commercialized as a nonaqueous liquid electrolyte secondary battery intended for a portable device such as a portable cellular phone. This particular battery has a positive electrode, a negative electrode and a separator which contains a liquid electrolyte, wherein utilized are lithium cobalt oxide (LiCoO2) for the positive electrode, a graphitic or carbonaceous material for the negative electrode, an organic solvent in which a lithium salt has been dissolved for the nonaqueous liquid electrolyte and a porous membrane for the separator.
The nonaqueous liquid electrolyte is a nonaqueous solvent in which an electrolyte has been dissolved, for which used usually is a low viscosity and low boiling point material such as a liquid mixture comprising propylene carbonate, ethylene carbonate or γ-butyrolactone, etc.
In the meantime, the nonaqueous liquid electrolyte secondary battery is adapted to be mounted on a portable device as being housed in a hermetically sealed container or the like. In so doing, a problem arises in which the nonaqueous liquid electrolyte may leak out of the hermetically sealed container. The battery also has a disadvantage that its electrodes may deform after charge/discharge cycles to largely swell its outer packaging.
To overcome such problems, a gel-like electrolyte has been proposed to ameliorate such leakage, which contains a solid electrolyte as an ion conducting material instead of using a nonaqueous liquid electrolyte. See, for example, Japanese Patent Unexamined Publication 2000-315523.
Since such a gel-like electrolyte contains a large amount of macromolecular material, however, the resin that is the base therefor will severely prevent the movement of the electrolyte, thereby remarkably reducing the ion conductivity as compared with the case of a nonaqueous liquid electrolyte alone. There has also been a disadvantage that contact with the electrodes will lessen as compared with the case of a liquid, which increases the resistance at the interface of the electrodes, thereby deteriorating the discharge characteristics of the nonaqueous liquid electrolyte secondary battery.
Furthermore, since a gel-like electrolyte is produced by encapsulating an electrolyte and a monomer together with electrodes in an outer packaging for the battery and then filling it with a gelling agent, the electrolyte will become gel-like while the gelling agent infiltrates from around the outer packaging toward the center. As such, there is a disadvantage that a homogeneous gel electrolyte may not easily be obtained between the electrodes and the electrodes per se will considerably deform during the charge/discharge cycles to swell the outer packaging.
As described above, a nonaqueous liquid electrolyte secondary battery utilizing a gel-like electrolyte containing a nonaqueous liquid electrolyte in order to prevent leakage or the like has been unable to provide sufficient discharge characteristics. In addition, a considerable amount of deformation will result in association with the charging and discharging